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eHALOPH. A database of salt tolerant plants.

The sensitivity of most higher plants to common salt, NaCl, is somewhat odd: most are killed by half the concentration of salt found in seawater. This sensitivity is peculiar as green plants evolved in the oceans, which cover about 70 % of the surface of the earth and now contain about 35 g per litre of NaCl. Plants that are not algae (for example, liverworts, mosses, ferns and seed plants) arose from a group of green algae about 490 million years ago, when the oceans were already salty, containing roughly 27 g NaCl per litre. This transition - from water to dry land - most probably occurred at the edges of fresh-water pools, as evaporation from the edges of salt-water pools would have made them too salty for any plants to survive. If, then, the first land plants originated at the edges of fresh-water pools, these organisms would have been adapted to living in fresh- rather than salt-water. Todays 351,000 known species of flowering terrestrial plants evolved already adapted to fresh water. Yet within this mass there are a small cluster, around 1500 species, of terrestrial plants that are salt-tolerant.

It has long been known that some plants can tolerate seawater – the fringes of the oceans have their own specialised flora called halophytes. However, it was not until the early twentieth century that halophytes were defined scientifically and research began on what allowed these plants to survive in salt concentrations that killed most other flora. During the 1980s, James Aronson began making a list of salt-tolerant plants. He deliberately chose a relatively low concentration of salt as the threshold for inclusion since he was interested in plants that might have value in agriculture, horticulture or floriculture in salty soils. So, if plants could tolerate about a quarter of the concentration of salt in seawater they were included in his list. This list was published in book form as “HALOPH, a Database of Salt Tolerant Plants of the World” in 1989.1 James Aronson included information, where available, on plant type, life form, maximum salinity tolerated, photosynthetic pathway, economic uses and geographical distribution. While the list of salt-tolerant plants was invaluable, its uses were limited by its format. So, over the period 2002 to 2005 the data was digitised and developed into a searchable electronic database, eHALOPH. Subsequently, new information was included with fields providing data on chemical composition (antioxidants, secondary metabolites and compatible solutes) and habitat, and whether or not there were publications on ecotypes, germination, the presence or absence of salt glands, molecular data, microbial interactions and mycorrhizal status. The database2 is now easily accessible over the internet (https://www.sussex.ac.uk/affiliates/halophytes/) and is being continually updated from published literature. Anyone can register and contribute; their contribution is assessed by the Administrator. There are currently about 1386 species listed in eHALOPH and of these just 525 or so can survive in 2/3rd the concentration of salt in seawater or above. These most tolerant halophytes (often called euhalophytes) make up just 0.15% of all flowering plants: they are exceptionally rare. 

So, why is eHALOPH important? Almost all of our current crop species are salt-sensitive and so are susceptible to salt and the process of salinisation. Salinisation of land occurs either when seawater inundates land or when irrigation over many years is mismanaged and salt gradually builds up in the soil. Salinisation is likely to be exacerbated by climate change as it threatens increased drought from the current levels which will increase the need for irrigation as well as inundation of coastal areas, due to a rise in sea level. Climate change clearly threatens our food supply.

The threat of salinisation for food supply was recognised in the mid-twentieth century and attempts were made, over the years, to breed crop varieties with salt-tolerance. However, this long-envisioned need to generate salt-tolerant crops has resulted in little progress, largely due to a lack of knowledge. Understanding the mechanism(s) by which plants tolerate salt - the physiology and biochemistry of halophytes - is a vital step towards generating salt-tolerant crop plants for the future. Halophytes also offer potential as crops themselves, particularly for forage and bioenergy and for phytoremediation of salt-affected areas. Ecological restoration and rehabilitation of degraded ecosystems may emerge as being as, or more, important than biosaline agriculture itself. eHALOPH is a resource to aid both goals. According to the UN Atlas of the Oceans approximately 44% of people live in coastal hinterlands.3

Climate change and salinisation

The atmospheric concentration of CO2 continues to rise (about 10% between 1990 and 2010);4 at a time when the global human population is also increasing (about 30% over the same period). Projecting future changes in climate is complex, but there is now little doubt that temperatures will continue to increase and rainfall will tend to be more unevenly distributed both spatially and temporally. In Europe, the effects will range from flash flooding across the North to frequent droughts in much of the South. Globally, the changes in climate and population combined with on-going economic growth and increasing consumption patterns will, by the 2050s, increase global food demand by c70-100% and global energy demand by c30-40%. Since the demand for energy can be partially met from plant biomass, energy and food production will compete for the same limited resources, challenging our ability to produce sufficient food and fibre on limited land and with diminishing mineral nutrients and water. 

Two statements from the Fifth Assessment Report of the Intergovernmental Panel on Climate Change demonstrate the timeliness of the development of eHALOPH and its potential to aid research into future food production: “Climate change has negatively affected wheat and maize yields for many regions and in the global aggregate (medium confidence)” and "In presently dry regions, drought frequency will likely increase by the end of the 21st century under RCP8.5 (medium confidence)".

An increase in aridity with high rates of evapotranspiration will concentrate soluble salts in the soil and surface water bodies and as such, increase the risk of salinisation. This risk will be further exacerbated by increased use of irrigation as, worldwide, 11% of the irrigated area is affected by salinity.6 Salinisation reduces not only agricultural productivity but also biological diversity and the water quality for urban and industrial use. In addition, there will be direct salinisation following inundation with seawater in coastal areas.

Over the period from 1972 to 2008, the global mean sea level has risen at the rate of about 2 mm y-1. Predicted increases in the sea level by 2100 vary between 0.3 to 1.1 m under a ‘business as usual’ scenario. While such a change may be small in relation to daily fluctuations in tidal height, it is likely to cause considerable impact over the long term (centuries) even if global warming is mitigated. Furthermore, modelling suggests there could be variations in different regions of the world by a factor of two in the 20-year average sea level rise from the period 1980–1999 to 2041–2060. A rise in sea level as well as increases in wave height and mean wind speed will influence the frequency of overtopping of sea defences and, as such, the frequency of exposing the coastal hinterland to saline water. Since the oceans of the world cover about 71% of the world’s surface area with some 356,000 km of coastline, the area of immediate hinterlands are the first in line for any inundation that might be caused by tsunami, storm surges or extremes in tidal height. Such events have occurred over the centuries and are expected to continue, or occur with greater frequency, as a consequence of global warming. The overtopping of coastal defences causes:

  1. increased flooding of low-lying areas with seawater and consequent salinisation of natural and agricultural lands, impacting on large populations (humans, other animals and plants).
  2. intrusion of seawater into rivers and aquifers.
  3. increased erosion and changes in sedimentation in estuaries.

Estimates are that 1.32 to 1.25 x 106 km2 and more than 700 million people may be affected. As many of these people depend on agriculture for their livelihood, so one aspect of adaptation might be the need to change the nature of the crops being grown in these areas so that they are more tolerant to salinity and waterlogging. eHALOPH lists species that are of value, directly or through research on the mechanisms of salt tolerance in plants, in mitigating the adverse effects of salinisation on agricultural and natural plant communities. Providing a valuable and vital resource for continued research and modelling against the future effects of climate change.

UK halophytes

The list below came from “Flowers of the Seaside” by Ian Hepburn published by Collins in 1954 and a Field Studies Council page on ‘The Seashore’ https://www.theseashore.org.uk/theseashore/saltmarsh%20section/species/Species%20information.html

The names are from Kew’s Plants of the World Online http://www.plantsoftheworldonline.org/

Species in grey are not currently in eHALOPH https://www.sussex.ac.uk/affiliates/halophytes/index.php

The list is in order of Family, then genus then species.

View the list here

End Notes

Go to footnote reference 1.

Aronson JA. 1989. Salt-tolerant plants of the world. Tucson: University of Arizona.

Go to footnote reference 2.

Santos J, Al-Azzawi M, Aronson J, Flowers T J (2016) eHALOPH a Database of Salt-Tolerant Plants: Helping put Halophytes to Work. Plant and Cell Physiology 57 (1) doi:10.1093/pcp/pcv155.

Go to footnote reference 3.

UN Atlas of the oceans http://www.oceansatlas.org/

Go to footnote reference 4.

IPCC 2015 Climate Change 2014: Synthesis Report. Contribution of Working Groups I, II and III to the Fifth Assessment Report of the Intergovernmental Panel on Climate Change. Geneva Switzerland: IPCC

Go to footnote reference 5.

ibid.

Go to footnote reference 6.

FAO, 2011. The state of the world’s land and water resources for food and agriculture (SOLAW) - Managing systems at risk. Rome and London: The Food and Agriculture Organization of the United Nations and Earthscan.

Go to footnote reference 7.

https://www.cia.gov/library/publications/the-world-factbook/

Professor T.J. Flowers

Tim Flowers began his research on halophytes more than 50 years ago and established, with others, the cellular basis of salt tolerance in plants. Subsequently, he and his colleagues worked at the University of Sussex for many years on methods for developing salt-resistant crops plants, including rice, chickpea and tomato. All of this research was carried out with overseas partners in Australia, China, Egypt, India, Pakistan, Spain and Syria. The research on rice led to an understanding of the complexity of salt tolerance as a trait and the development of a view on how tolerance might be improved. In spite of some success with generating salt tolerant genotypes of rice, Professor Flowers advocated the domestication of halophytes as a more cost-effective way of generating at least some salt-tolerant crops. He has, over the years, held posts including the Dean of the School of Biological Sciences at the University of Sussex, made many overseas visits in connection with his research and has supervised or co-supervised 35 postgraduate students. He is the author of 150 scientific papers.